1. Field of the Invention
This disclosure is related to the field of audio acoustics. More specifically, to sound reproduction using an acoustic driver or transducer and a complimentary acoustic system such as an enclosure or housing for the driver to produce a loudspeaker capable of reproducing and emphasizing sub-bass tones.
2. Description of the Related Art
The reproduction of bass tones through audio systems mounted in homes or automobiles is a well-established technology, but it is also recognized that producing low tones, commonly called deep bass or sub-bass, can be extraordinarily difficult. Systems for performing such reproduction which will generally focus on tones of less than 40 Hz, less than 20 Hz, and even down to single digit ranges, generally share a few key features. The primary shared feature is that bass speakers that produce particularly low tones are often very large. In effect, the larger the cone of the speaker, the better able it is to handle lower tones. By extension, such systems are also generally quite expensive.
Sub-bass sounds (and even sub sub-bass or first octave tones) are particularly important in pipe organ music (where a large pipe organ can produce exceedingly low tones potentially into the single digit Hz range and commonly below 20 Hz), certain types of music featuring well played low bass instruments (such as the tuba which can also produce tones well below 20 Hz in the hands of a skilled player), many forms of modern dance club music (where tones of any value can be produced electronically and particularly low tones are commonly used to provide for “feel” to the music), and musical works that utilize uncommon instrumentation (for example the cannons in Pyotr Tchaikovsky's 1812 Overture).
It is commonly accepted that tones below 20 Hz are not actually capable of being heard by a human being, however, that does not mean these tones are unimportant in music reproduction. For some types of audio enthusiasts, the production of bass tones is a physical thing. The notes are more felt than heard, and the purpose of their reproduction is not necessarily as much to provide for audio depth and rhythm, as it is to provide raw force. This can be common in dance clubs where a pulsing beat of music is more felt than heard in the club with the bass literally vibrating structures and bodies. This type of physical bass “thump” is also commonly used in mobile audio applications where production of such tones can serve to shake the car providing both feel and potentially a desirable “rattle” from the cars body. While many audio enthusiasts will shun this type of audio reproduction as making the bass of the music heavy or over-emphasized (essentially contending that the music is distorted), there is a clear group which both enjoys this sensation, and there is a need for it in certain types of recorded music to accurately reproduce not just the music's sound but its feel.
Deep bass production can also be very important in areas other than in music. In movies for example, deep, potentially inaudible bass reproduction can be necessary to produce mood. The movie Jurassic Park includes a particularly well known scene where steps of an approaching Tyrannosaurs Rex are felt (and seen in a rippling cup of water) rather than heard. Similarly, images of scenes of earthquakes or natural disasters can be emphasized by providing an audio track with a physical component (where the shaking is quite literally felt) in addition to a sonic one.
There exist some acoustic waveguides and audio transmission lines that enhance or extend the range of a given transducer. One such device is shown in U.S. Pat. No. 4,628,528. In this device, improved bass reproduction is provided through the use of two acoustic waveguides coupled to the front and rear of a loudspeaker driver. The system utilizes mounting the driver on an acoustic baffle to isolate forward and rearward deflections of the driver cone. Forward deflections are channeled through an aperture of substantially the same diameter as the cone through a short acoustic waveguide, while rearward deflections are directed through a shorter waveguide (3 time the length of the forward one) to produce cavity resonance producing lower tones. These systems, however, are not generally expected to produce particularly low sub-bass tones (e.g. below 40 Hz) with any substantial volume.